Introduction to Specific Heat

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Objective

SWBAT use experimental data to compare how four substances given the same amount of heat energy change temperatures at different rates.

Big Idea

Specific heat connects the amount of heat, the mass, and the temperature change of a substance.

Background

After grading the previous day's papers, I knew students needed more work with the idea of specific heat.  Since specific heat shows the relationship between different properties, I thought that a graphing activity would be a good choice so students could connect the rate of temperature change with the specific heat.

This activity was found by a colleague and shared in our district last year.  I do not know the original source to give proper credit.  It is an excellent activity that connects graphing and finding patterns in data to understand how specific heat works.

This lesson is rooted in sample data and mathematical understandings.  As such, it connects to  Science and Engineering Practice 4, analyzing and interpreting data, and Science and Engineering Practice 5, using mathematical and computational thinking.  It also aligns with the Energy and Matter Cross Cutting Concept: Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system.  This lesson continues our exploration of HS-PS3-4, plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperature are combined within a closed system results in a more uniform energy distribution among the components in the system.

Opener: Connecting Concepts

10 minutes

When students enter, I am passing back their "Heat, Temperature and Calorimetry" packets from the day before.  I compliment the students on the overall job they did in completing the work with a partner.

To review, I write the q=mcΔT equation on the board.  I ask the class what they remember from the previous day about each variable.  As they provide information, I record it on the board.

  • m is the mass
  • What are the units for mass?
  • g, for grams.
  • ΔT is the temperature
  • Is it only the temperature?  Why does it have a delta sign, or a triangle?
  • Its the change in temperature
  • Which means what?
  • The end temperature minus the original temperature
  • What is q?
  • The total heat.
  • How is it measured?  Most of you left this off your practice calculations yesterday.
  • Joules, or J
  • Now c might be hard to remember, anyone got it?
  • Specific heat.
  • Why do they use "c" for it?
  • It's the specific heat Capacity, so they abbreviate it with a c.  What does it mean?
  • <crickets>
  • This tells us how much heat a sample can absorb before changing temperature.  What changes temperature faster, equal amount of metal or water?
  • Metal.
  • Because it has a lower capacity for heat, so when it gets heat, it can't hold it without changing temperature.

We finish with the equation labeled like this:

I then relate the high specific heat capacity of water to how it is "cooler by the lake (Michigan)" during the summer at the beach.   I tell the story of how I went to Indiana Dunes in mid-May once in college, to be shocked that the water was still 58 degrees even though it was 80 degrees out that day.

Data Graphing

10 minutes

As I wrap up my story, I explain that we are going to use a sample data set to expand our understanding of how specific heat works, and I pass out the day's assignment.  I explain the set up, that there are four equal mass samples of different substances: water, sand, air and metal left out in the sun for an equal amount of time.  The temperature was measured every 15 minutes to see how the substances heated up in the sun.  Students need to create a multi-line graph showing the heating of each substance in a different color, and then answer the questions based upon their graphs.

Students are not thrilled to be making graphs again, but this time, given the axes and scales, do much better than they had a month ago.  Here are two samples, and the answers to the first two questions.

I love how the student shaded the data table labels to make a simple key for the graph.  This student makes a common mistake on question number 2.  Many students assumed that the water would cool faster because it begins at the lowest temperature, rather than thinking of cooling as the opposite of heating.

Once the graph is made, students begin to work on the questions.  If they form pairs, I allow it, although many work alone, just checking in with a classmate if they get stuck.

Data Analysis

30 minutes

As students work on the analysis, they may have difficulty based on their understanding of specific heat.  The students who get it inherently fly through these questions and have answers like these:

The only error in this work is on number four, where the student later added "different masses" although the setup indicated that all the samples had 10.0g.

Many students made an initial error on number 5, believing that the tallest graph would have the highest heat capacity.  As I circulated the room, I talked students through this mistake.  About half way through the day, I found a simple demonstration to explain it to students that was very effective.

When students wrap up the questions, I ask them to try the word problems on the back side of the paper.  I don't expect them to complete these problems, but have them there as an extension for students who are fast in their graphing and analysis.  When viewing the student work, I was disappointed at the lack of units on answers, but didn't stress it in the instructions, so I make a note to mention it when we do practice problems.  We did not teach significant figures this year, so the answers are pure calculator results.